JP7734100B2 - Mesh network system and mobile node - Google Patents

Description

The present invention relates to a mesh network system having airborne mobile nodes connected to a wireless network and multiple fixed nodes that form the wireless network and are also connected to a wired network.

Recent advances in robotics technology have been remarkable, and robots are increasingly being used to solve a variety of social issues. Many of these robots are unmanned mobile objects, such as unmanned aerial vehicles and autonomous vehicles. Unmanned mobile objects need to be equipped with a communications system to transmit control command data for remote control and autonomous control, as well as video data captured by cameras mounted on the unmanned mobile object. In such cases, if the unmanned mobile object is not a machine that moves along a predetermined path such as a rail, wireless communications suitable for movement are often used.

For example, Patent Document 1 discloses an invention in which a mobile base station and a terminal station are equipped with a long-distance communication function for preparing for short-distance communication and a short-distance communication function for data transmission, and the timing of short-distance communication is scheduled using communication using the long-distance communication function. Furthermore, Patent Document 2 discloses an invention in which, in a relay system using an unmanned aerial vehicle, the relay position of the unmanned aerial vehicle is searched for based on the relay communication quality, the planned relay time, and the power supply status (available power supply amount) of the unmanned aerial vehicle.

International Publication No. 2017/018021 Japanese Patent Application Laid-Open No. 2019-169848

One type of communication network that can be used to communicate with unmanned vehicles is a mesh network. A mesh network is made up of multiple nodes, and by connecting adjacent nodes that can communicate with each other, the entire network forms a mesh-like structure. In particular, when the connections between nodes are made wirelessly, it is called a wireless mesh network, and data is transferred wirelessly across multiple nodes in a bucket brigade style.

Mesh networks typically have multiple routes, and if communication becomes unavailable on one route, they can switch to an alternative route, making them more resilient to failures than other network topologies. However, the existence of multiple routes also means that the network contains loop-shaped routes. When broadcasting IP packets on such a network, spanning tree protocols are generally used to prevent the IP packets from repeating broadcasts even after making a full loop around the looped route, causing a broadcast storm.

The spanning tree protocol uses a spanning tree algorithm to automatically block communication between specific nodes, making it possible to prevent loops. Furthermore, in the event of a failure, the blocking state can be lifted, allowing communication to continue via an alternative route.

However, such mesh networks are less efficient than centralized control methods because routes are determined by all nodes on an equal footing, rather than by centralized control by specific high-priority nodes. This means that highly fault-tolerant mesh networks may not be feasible, particularly in cases where high throughput and low latency are required.

One use case that requires high throughput and low latency in addition to fault tolerance is a wireless communication system that transmits video related to the remote control of an unmanned aerial vehicle (UAV). Figure 1 shows a configuration example of a conventional example 1 of such a wireless communication system. For example, when the UAV is flying beyond visual line of sight, this system transfers video captured from the UAV of the surrounding environment to a server, allowing the pilot to view the video in real time.

In the wireless mesh network of Conventional Example 1 shown in Figure 1, a first fixed node 111, a second fixed node 112, a third fixed node 113, and an nth fixed node 114 are placed on the ground along the flight path of a mobile node 101, including the UAV body. A server 121 is also connected to the first fixed node 111 by wire. In the following explanation, the number of hops is defined as the number of nodes between which communication takes place minus one, regardless of whether they are mobile or fixed nodes. For example, one hop is defined as wireless communication between two nodes, and two hops is defined as wireless communication between three consecutive nodes.

When the mobile node 101 is flying above the first fixed node 111, one-hop communication takes place between the mobile node 101 and the first fixed node 111, and data is transferred to the server 121 via the shortest route. Next, when the mobile node 101 moves above the second fixed node 112 and is no longer able to communicate directly with the first fixed node 111, two-hop communication takes place from the mobile node 101 to the first fixed node 111 via the second fixed node 112. Similarly, when the mobile node 101 moves above the third fixed node 113, the communication becomes three-hop communication, and when it moves above the nth fixed node 114, the communication becomes n-hop communication. As the number of hops increases, throughput decreases and delay time also increases.

Furthermore, for example, if the first fixed node 111 and the third fixed node 113, which are located on either side of the second fixed node 112, cannot detect each other's carrier, there is a risk that a packet transmitted by the first fixed node 111 and a packet transmitted by the third fixed node 113 will collide at the second fixed node 112. In this way, when multiple nodes that are unable to detect each other's communications simultaneously transmit signals to the same node, the signals may collide (the so-called hidden terminal problem), further reducing throughput.

Figure 2 shows an example configuration of a wired and wireless mesh network (hereinafter referred to as a "wired/wireless mesh network") as Prior Art Example 2. In the system of Figure 2, similar to the system of Figure 1, a first fixed node 211, a second fixed node 212, a third fixed node 213, and an nth fixed node 214 are placed on the ground along the flight path of a mobile node 201 including the UAV body, and a server 221 is further connected to all of the fixed nodes by wire. However, wireless communication between fixed nodes is prohibited.

Specifically, the MAC addresses of the second fixed node 212, the third fixed node 213, and the nth fixed node 214 are specified for the first fixed node 211, and MAC address filtering is used to prohibit connection with these fixed nodes. This allows the first fixed node 211 to wirelessly connect only with the mobile node 201. Similar settings are made for the other fixed nodes.

The system in Figure 2 also differs from Conventional Example 1 in that the maximum number of hops is set to 1. This is achieved by setting the TTL (Time To Live) in the IP header of the packet to be sent to 1. When a packet is transferred from wireless node to wireless node, 1 is subtracted from the TTL, immediately setting it to 0. If the packet has not reached the destination node at this point, it is discarded, resulting in a wireless network with a maximum number of hops of 1. In other words, because the route shown by the dashed line in Figure 2 can only be traversed once, no loops are formed even if the wireless network and wired network are connected by multiple nodes, and no blocking occurs between a specific fixed node and server 221.

In this way, if the wireless network exceeds a certain number of hops, packets are forcibly discarded to prevent the formation of a loop, thereby preventing blocking on the wired network. Furthermore, by setting a route that always ensures that the wired network can be reached from the wireless network within a certain number of hops, communication is always carried out via the shortest route, regardless of which fixed node the mobile node 201 is connected to. This makes it possible to build a network that achieves high throughput and low latency.

However, for example, if there is an external communication device that can detect the carrier between the second fixed node 212 but cannot detect the carrier between the mobile node 201, there is a risk that packets sent by the mobile node 201 and packets sent by the external communication device will collide at the second fixed node 212. In other words, even in the system of Conventional Example 2 shown in Figure 2, a decrease in throughput due to the hidden terminal problem may occur.

The present invention was made in consideration of the above-mentioned conventional circumstances, and aims to provide a mesh network system that can suppress the decrease in throughput due to the hidden terminal problem.

In order to achieve the above object, a mesh network system according to one aspect of the present invention is configured as follows.
That is, in a mesh network system having an airborne mobile node connected to a wireless network and a plurality of fixed nodes that form the wireless network and are also connected to a wired network, each of the plurality of fixed nodes is characterized by using an antenna pattern with reduced horizontal gain so that carriers cannot be detected between adjacent fixed nodes.

Here, in the mesh network system according to the present invention, each of the multiple fixed nodes may use an antenna pattern in which the gain in the sky direction is higher than the gain in the horizontal direction.

Furthermore, in the mesh network system according to the present invention, the mobile node may use an antenna pattern in which the gain in the ground direction is higher than the gain in the horizontal direction.

Furthermore, a mobile node according to another aspect of the present invention is configured as follows.
That is, in an airborne mobile node connected to a wireless network made up of multiple fixed nodes connected to a wired network, the node has a first antenna pattern in which the horizontal gain is higher than the ground gain, and a second antenna pattern in which the horizontal gain is lower than the first antenna pattern while the ground gain is higher, and is characterized by switching to use the first antenna pattern when the flight altitude is below a predetermined threshold, and to use the second antenna pattern when the flight altitude is equal to or higher than the threshold.

The present invention provides a mesh network system that can mitigate the reduction in throughput caused by the hidden terminal problem.

FIG. 1 is a diagram illustrating a configuration example of a wireless mesh network according to a first conventional example. FIG. 10 is a diagram illustrating a configuration example of a wired/wireless mesh network according to Conventional Example 2. 1 is a diagram illustrating an example of the configuration of a wired/wireless mesh network according to an embodiment of the present invention. FIG. 10 is a diagram illustrating an example of the configuration of a mobile node according to another embodiment of the present invention.

An embodiment of the present invention will be described below with reference to the drawings.
FIG. 3 shows an example of the configuration of a wired/wireless mesh network according to an embodiment of the present invention. The wired/wireless mesh network of this example includes a mobile node 301, a first fixed node 311, a second fixed node 312, a third fixed node 313, an nth fixed node 314, and a server 321. The mobile node 301 is mounted on an unmanned UAV (unmanned aerial vehicle) or the like. The fixed nodes 311-314 are placed on the ground, for example, along the flight path of the UAV. The mobile node 301 and the fixed nodes 311-314 are wireless nodes capable of wireless communication with each other. However, wireless communication between the fixed nodes is prohibited. All of the fixed nodes 311-314 are connected to the server 321 via a wired network.

The mobile node 301 uses an antenna pattern with a high gain in the direction toward the ground. Here, the mobile node 301 uses an antenna pattern with a higher gain in the direction toward the ground than in the horizontal direction.

Furthermore, all fixed nodes 311 to 314 use an antenna pattern with high gain in the sky direction and low gain in the horizontal direction. Here, fixed nodes 311 to 314 use an antenna pattern that suppresses gain in the horizontal direction so that carriers cannot be detected between adjacent fixed nodes, and in which gain in the sky direction is higher than gain in the horizontal direction.

The antenna patterns of the mobile node 301 and the fixed nodes 311-314 can be achieved by providing each node with a dedicated antenna appropriate for its role. Alternatively, they may be achieved by providing each node with a mechanism capable of controlling the antenna pattern used. As one example, each node may be provided with a first antenna with a higher horizontal gain than a vertical gain and a second antenna with a higher vertical gain than a horizontal gain, and the desired antenna pattern may be used by switching between the antennas depending on the node's role. As another example, each node may be provided with either the first or second antenna described above, and the desired antenna pattern may be used by changing the orientation of that antenna. As yet another example, each node may be provided with an antenna unit with controllable antenna directivity, and the desired antenna pattern may be used by adjusting the direction and width of the transmitting and receiving beams to change the antenna directivity.

The operation of the wired/wireless mesh network of this example will be explained below with reference to Figure 3. As shown in Figure 3, all fixed nodes 311-314 are not only arranged along the UAV's flight path, but are also positioned so that their antenna patterns with high gain in the sky overlap in the sky, allowing mobile node 301 to communicate with at least one or more fixed nodes along the UAV's flight path. On the other hand, the antenna patterns of all fixed nodes 311-314 have low gain in the horizontal direction, and are positioned so that carriers cannot be detected between adjacent fixed nodes.

In addition, in this example's wired/wireless mesh network, the maximum number of hops is set to 1. This can be achieved by setting the TTL (Time to Live) in the IP header of the packet to be sent to 1. In this case, when a packet is transferred from wireless node to wireless node, 1 is subtracted from the TTL, immediately setting it to 0. If the packet has not reached the destination node at this point, it is discarded, resulting in a wireless network with a maximum number of hops of 1.

Furthermore, in this example of a wired/wireless mesh network, the fixed nodes 311-314, which are connected to both the wireless and wired networks, are equipped with MAC address filtering functionality to prevent loops from forming between the wireless and wired networks. Specifically, when a packet with a source address belonging to the wireless network is received from the wired network, the packet is discarded, thereby avoiding broadcast storms without blocking the route on the wired network.

For example, when a packet sent from mobile node 301 belonging to a wireless network reaches second fixed node 312 and then reaches other fixed nodes 311, 313, and 314 via the wired network, the packet is discarded. In contrast, if a packet sent from server 321 belonging to a wired network reaches second fixed node 312 and then reaches mobile node 301 via the wireless network, the packet will not be forwarded to other fixed nodes because the wireless network has a maximum hop count of 1. Note that fixed nodes connected to both wireless and wired networks have different MAC addresses on the wireless and wired sides.

Here, assume that mobile node 301 is flying above first fixed node 311, and that only first fixed node 311 is wirelessly connected to mobile node 301. When mobile node 301 sends data such as video information to server 321 for the first time in this state, the IP address of server 321 is usually known, but the MAC address of server 321 is unknown, so an ARP request packet is broadcast. The ARP request packet reaches server 321 only via the route via first fixed node 311, and address resolution is achieved when server 321 returns its own MAC address in an ARP response packet to mobile node 301. On the other hand, ARP request packets that reach other fixed nodes 312, 313, and 314 from first fixed node 311 via the wired network are discarded by the MAC address filtering function.

After address resolution, data from mobile node 301 is transmitted wirelessly in one hop to first fixed node 311, and then forwarded via wired connection to server 321. Incidentally, Figure 3 shows that there is a house near first fixed node 311, and interference waves of the same frequency are leaking from the windows of this house. Note that first fixed node 311 has an antenna pattern with low gain in the horizontal direction, making it less likely that packets from mobile node 301 and packets from the house will collide at first fixed node 311.

Similarly, collisions between packets from the mobile node 301 and packets from the second fixed node 312 are less likely to occur at the first fixed node 311. Furthermore, because the interference waves leaking through the windows of the houses have a low level of radiation into the sky, if the flying altitude of the mobile node 301 is sufficiently high, collisions between packets from the first fixed node 311 and packets from the houses are less likely to occur at the mobile node 301. Although not explicitly shown in Figure 3, interference waves from nearby houses may exist at other fixed nodes, just like the first fixed node 311. However, because all fixed nodes have antenna patterns with low gain in the horizontal direction, it is expected that packet collisions will be reduced, just like the first fixed node 311.

Next, as the mobile node 301 approaches the sky above the second fixed node 312, a wireless connection is established between the mobile node 301 and the second fixed node 312, and the mobile node 301 becomes connected to the first fixed node 311 and the second fixed node 312. If the mobile node 301 subsequently moves further and is no longer able to communicate with the first fixed node 311, the mobile node 301 again broadcasts an ARP request packet to the server 321. This ARP request packet reaches the server 321 via a route that passes through the second fixed node 312, and the server 321 returns an ARP response packet, causing address resolution to be performed again.

Similarly, as the mobile node 301 approaches the sky above the third fixed node 313, a wireless connection can be established between the mobile node 301 and the third fixed node 313. Thereafter, when the mobile node 301 moves further and is no longer able to communicate with the second fixed node 312, the mobile node 301 again broadcasts an ARP request packet to the server 321. This ARP request packet reaches the server 321 via a route that passes through the third fixed node 313, and the server 321 returns an ARP response packet, causing address resolution to be performed again.
In this way, similar route switching control is performed until the mobile node 301 reaches the nth fixed node 314 located near the final destination.

As described above, the mesh network system of this example comprises an airborne mobile node 301 connected to a wireless network, and multiple fixed nodes 311-314 that form part of the wireless network and are also connected to a wired network. Each of the multiple fixed nodes 311-314 uses an antenna pattern with reduced horizontal gain so that adjacent fixed nodes cannot detect carriers. This makes it possible to avoid packet collisions between fixed nodes, thereby minimizing the reduction in throughput due to the hidden terminal problem. Furthermore, by reducing horizontal gain, it becomes possible to suppress interference from surrounding residential areas, etc.

Furthermore, in this example mesh network system, each of the multiple fixed nodes 311-314 uses an antenna pattern in which the gain in the sky direction is higher than the gain in the horizontal direction. This allows each fixed node 311-314 to communicate stably with the mobile node 301 flying overhead.

Furthermore, in this example mesh network system, the mobile node 301 uses an antenna pattern in which the gain in the ground direction is higher than the gain in the horizontal direction. This allows the mobile node 301 to maintain a flight altitude that minimizes the effects of interference from residential areas, while still enabling stable communication with the fixed nodes 311-314 on the ground.

Furthermore, in this example mesh network system, blocking on the wired network side is prevented by preventing loops from forming through the setting of the maximum number of hops on the wireless network side and the setting of MAC address filtering on the wired network side. This makes it possible to build a network that is resilient to failures while maintaining high throughput and low latency.

Figure 4 shows an example of the configuration of a mobile node according to another embodiment of the present invention. The mobile node 401 in this example is a wireless node mounted on an unmanned UAV (unmanned aerial vehicle) and can be used in place of the mobile node 301 described above. The mobile node 401 includes a first antenna 411, a second antenna 412, an antenna switch 413, a wireless processing unit 414, and an altimeter 415.

The first antenna 411 is an antenna having a first antenna pattern in which the gain in the horizontal direction is higher than the gain in the ground direction. The second antenna 412 is an antenna having a second antenna pattern in which the gain in the horizontal direction is lower than that of the first antenna pattern, while the gain in the ground direction is higher (for example, an antenna pattern in which the gain in the ground direction is higher than the gain in the horizontal direction). The wireless processing unit 414 performs processing related to wireless communication using the first antenna 411 or the second antenna 412. The altimeter 415 is a sensor that measures the flight altitude of the mobile node 401.

The antenna switch 413 switches the antenna to be used for wireless communication based on the flight altitude measured by the altimeter 415. In this example, when the flight altitude is below a predetermined threshold, the first antenna 411 is connected to the wireless processing unit 414. In other words, when the flight altitude is low, a first antenna pattern in which the horizontal gain is higher than the gain toward the ground is used for wireless communication. On the other hand, when the flight altitude is equal to or higher than the predetermined threshold, the second antenna 411 is connected to the wireless processing unit 414. In other words, when the flight altitude becomes relatively high, the antenna is switched so that a second antenna pattern in which the horizontal gain is lower than the first antenna pattern but the gain toward the ground is higher is used for wireless communication.

In this way, by switching antennas taking into account the changing positional relationship with the fixed node depending on the flying altitude of the mobile node, wireless communication with the fixed node can be carried out more stably. Note that while the example in Figure 4 is configured to switch between two antennas, three or more antennas may also be provided that can be switched. Furthermore, rather than switching between multiple physically different antennas, an antenna whose antenna directivity can be dynamically changed (for example, an array antenna) may also be used.

The above describes embodiments of the present invention, but the above embodiments are merely examples and do not limit the technical scope of the present invention. The present invention can take on a variety of other embodiments, and various modifications such as omissions and substitutions can be made without departing from the spirit of the present invention. These embodiments and their modifications are included within the scope and spirit of the invention described in this specification, etc., and are also included in the inventions described in the claims and their equivalents.

In addition, the present invention can be provided not only as the devices described above or as systems composed of these devices, but also as methods executed by these devices, programs for implementing the functions of these devices using a processor, and storage media for storing such programs in a computer-readable format.

The present invention can be used in a mesh network system that has airborne mobile nodes connected to a wireless network and multiple fixed nodes that form the wireless network and are also connected to a wired network.

101, 201, 301, 401: Mobile nodes, 111, 211, 311: First fixed node, 112, 212, 312: Second fixed node, 113, 213, 313: Third fixed node, 114, 214, 314: Nth fixed node, 121, 221, 321: Server, 411: First antenna, 412: Second antenna, 413: Antenna switch, 414: Radio processing unit, 415: Altimeter

Claims (4)

A mesh network system having a mobile node mounted on an unmanned flying vehicle and connected to a wireless network, and a plurality of fixed nodes that constitute the wireless network and are also connected to a wired network,
the maximum hop count of the wireless network is 1;
each of the plurality of fixed nodes has a MAC address filtering function for discarding a packet when the packet has a source address belonging to the wireless network and is received from the wired network side;
Furthermore, the mesh network system is characterized in that each of the multiple fixed nodes uses an antenna pattern that reduces horizontal gain regardless of whether or not the external communication equipment is present, so that carriers are not detected between adjacent fixed nodes and so that packets from the mobile node and packets from external communication equipment located at a position lower than the flight path of the unmanned mobile body do not collide at the fixed node. 2. The mesh network system according to claim 1,
A mesh network system, wherein each of the plurality of fixed nodes uses an antenna pattern having a higher gain in the sky direction than in the horizontal direction. 3. The mesh network system according to claim 1,
A mesh network system characterized in that the mobile node uses an antenna pattern in which the gain in the ground direction is higher than the gain in the horizontal direction. 3. The mesh network system according to claim 1,
The mobile node:
a first antenna pattern having a horizontal gain higher than a ground gain;
a second antenna pattern that has a lower gain in the horizontal direction and a higher gain in the ground direction than the first antenna pattern,
A mesh network system characterized by switching to use a first antenna pattern when the flight altitude is below a predetermined threshold, and to use a second antenna pattern when the flight altitude is above the threshold.